Pseudo halftone image processing using a threshold matrix to form dots not rotational symmetric on a 90 degree basis

ABSTRACT

An image processing apparatus which performs image processing for print processing, including: a pseudo halftone processing unit configured to perform pseudo halftone processing by dithering with respect to an input image, and generate a halftone image constituted by plural dots, and a threshold matrix holding unit configured to hold a threshold matrix used for the pseudo halftone processing, wherein in the threshold matrix, thresholds are arranged such that the halftone image has a density region in which dots to be formed depending on a density of the input image are not rotational symmetric on a 90° basis, and orientations of the dots differ from each other in the density region.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to pseudo-halftoning processing,and more particularly, to a technique in consideration of a relationshipbetween a shape of a dot formed by dithering and image rotationprocessing.

Description of the Related Art

Various types of image recording systems are used in image formingapparatuses, such as copiers and printers. For example, in anelectrophotographic system, a latent image is formed on aphotoconductive drum using a laser beam, the latent image is developedwith a charged coloring material (hereinafter, “toner”), a developedtoner image is transferred to and fixed to a paper sheet, whereby animage is recorded. Such an electrophotographic image forming apparatusoften can output at a small number of tones in pixel units. Therefore,in order to stably and accurately reproduce halftone expression of imagedata of a printing target, image data with a large number of tones inpixel units is subject to pseudo halftone processing for expressingtones using plural pixels in order to reduce the number of tones inpixel units.

For the purpose of printing finish processing, such as bookbinding andsorting, or changing cassettes upon running out of paper sheets, imagedata may be stored in a storage device, such as memory or hard diskdrive, in the image forming apparatus, and then subject to rotationprocessing on a 90° basis before being printed out.

Regarding storage of image data, storing image data expressed by a smallnumber of tones which has been subject to the pseudo halftone processingis advantageous compared with image data with a large number of tonesfor small capacitance of the storage device and short writing time tothe storage device.

However, the related art image forming apparatuses have the followingproblems. If image data which has been subject to pseudo halftoneprocessing is printed out after performing rotation processing on a 90°basis, image density is changed as compared with a print output resultof image data which is not subject to rotation processing. This isbecause, in an image converted into a dot pattern corresponding to acertain specific density, vertical and horizontal orientations of thedot pattern are exchanged by rotation processing in thepseudo-halftoning processing, whereby a horizontally long dot is changedinto a vertically long dot, for example. A change in the dot patternproduces a change in a latent image pattern obtained by laserirradiation on a photoconductive drum in, for example, anelectrophotographic printing process. This is because rotation of theimage data means that a laser scanning direction and a rotationdirection of the photoconductive drum are exchanged. For example, it isassumed that a certain image forming apparatus has high reproducibilityof a horizontally long dot (i.e., a horizontally long dot is outputdenser than a vertically long dot with the same density value)corresponding to a specific laser scanning direction or a specificrotation direction of a photoconductive drum. In this case, a dotformation process is changed by the rotation of the image and if, forexample, a dot which is horizontally long before rotation is changedinto a vertically long dot, density of an image formed on a paper sheetat a transfer step is changed (i.e., becomes thinner in the exampleabove). A change in density caused by a change in the dot formationprocess accompanying rotation of the image may occur also in otherrecording methods (e.g., an inkjet recording method).

If the orientation of print output is known in advance, to whichdirection the image is to be rotated can be known in advance, andcreating a dot pattern converted in the pseudo-halftoning processing inconsideration of the rotational direction is also possible. For example,if it is known in advance that an image is printed out after beingrotated 90°, a printed matter of density equivalent to that withoutrotation can be obtained by forming a dot pattern rotated 270° even ifthe image is rotated 90° later. However, it is difficult to determinethe rotational direction in advance in all the cases.

Some image forming apparatuses, such as copiers and printers, areprovided with plural sheet feed cassettes for feeding paper sheets. Suchimage forming apparatuses provided with plural sheet feed cassettesoften have an automatic cassette change function. The automatic cassettechange function is a function to automatically switch to another sheetfeed cassette accommodating paper sheets of the same type when a certainsheet feed cassette runs out of paper sheets. This function reduces timeand effort of a user who switches setting of the sheet feed cassettewhich is a sheet feeding source. With the automatic cassette changefunction, for example, the paper sheet may be fed in a landscapedirection first and, may be changed to a portrait direction after thesheet feed cassette which is a sheet feeding source is changed uponrunning out of paper sheets. In this case, rotation processing of theimage data is required for the print output and the problem of thechange in density described above may occur. However, it is difficult toknow in advance the timing at which the sheet feed cassettes are changeddue to running out of paper sheets under a real usage environment.

To address the problem of density variation accompanying rotationprocessing of the image data which has been subject to pseudo halftoneprocessing, a technique of applying a dithering matrix with highrotation tolerance with respect to image data which has been subject topseudo-halftoning processing by dithering is proposed. Japanese PatentLaid-Open No. 2010-220145 describes forming a dot pattern with nodifference between a dot gain of an image at a rotation angle of 0° anda dot gain of the same image rotated 90°. Specifically, described is atechnique of performing pseudo-halftoning processing by dithering usinga dithering matrix for forming a dot in which the same continuous numberof pixels exist both in a scanning direction and in a sub-scanningdirection (i.e., a dot in which continuous number of pixels in certaindirections are rotational symmetric). With this technique, even if theimage data is subject to rotation processing after pseudo-halftoningprocessing, image data with equivalent density can be obtained beforeand after the rotation processing.

However, since the technique described in Japanese Patent Laid-Open No.2010-220145 has a restriction in a change in the size of the dotcorresponding to density, a linear change in density is difficult. Thisis because, although density is increased continuously typically byincreasing the size of the dot gradually, a dot shape which does notnecessarily follow the rule of the rotational symmetric described aboveis formed in a process of increasing the pixels constituting the dot(“ON pixels”) one by one. As a result, in a density region in which adot violating a rule of rotational symmetric is formed, a change indensity is caused accompanying the rotation processing. Alternatively,if control of a change in density is considered to be important and itis tried not to form a dot violating a rule of rotational symmetric,then the number of ON pixels must be increased at once in a process ofincreasing the size of the dot, whereby continuity of density andcontinuity of tone are impaired.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image processingapparatus which performs image processing for print processing,including: a pseudo halftone processing unit configured to performpseudo halftone processing by dithering with respect to an input image,and generate a halftone image constituted by plural dots, and athreshold matrix holding unit configured to hold a threshold matrix usedfor the pseudo halftone processing, wherein in the threshold matrix,thresholds are arranged such that the halftone image has a densityregion in which dots to be formed depending on a density of the inputimage are not rotational symmetric on a 90° basis, and orientations ofthe dots differ from each other in the density region.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a basic configuration of amulti-function printer (MFP).

FIG. 2 is a block diagram illustrating an internal configuration of animage processing unit.

FIG. 3 is a block diagram illustrating an internal configuration of anoutput image processing unit.

FIG. 4 is a flowchart illustrating a processing flow in the output imageprocessing unit.

FIG. 5A illustrates a conventional threshold matrix, FIG. 5B illustratesa resultant image of dither processing using the threshold matrix, andFIG. 5C illustrates an image which is the resultant image subject torotation processing.

FIG. 6A illustrates a threshold matrix with rotation tolerance accordingto a first embodiment, FIG. 6B is a resultant image of dither processingusing the threshold matrix, and FIG. 6C illustrates an image which isthe resultant image subject to rotation processing.

FIG. 7A illustrates a conventional threshold matrix, FIG. 7B illustratesa resultant image of dither processing using the threshold matrix, andFIG. 7C illustrates an image which is the resultant image subject torotation processing.

FIG. 8A illustrates a threshold matrix with rotation tolerance accordingto the first embodiment, FIG. 8B is a resultant image of ditherprocessing using the threshold matrix, and FIG. 8C illustrates an imagewhich is the resultant image subject to rotation processing.

FIG. 9 illustrates exemplary control of a threshold matrix so that ashape of a dot becomes rotationally symmetric at 180° when the totalnumber of pixels constituting the dot is an even number.

FIG. 10A illustrates a threshold matrix with rotation toleranceaccording to a second embodiment, FIG. 10B is a resultant image ofdither processing using the threshold matrix, and FIG. 10C illustratesan image which is the resultant image subject to rotation processing.

FIG. 11A illustrates a threshold matrix with rotation toleranceaccording to the second embodiment, FIG. 11B is a resultant image ofdither processing using the threshold matrix, and FIG. 11C illustratesan image which is the resultant image subject to rotation processing.

FIG. 12A illustrates a threshold matrix with rotation toleranceaccording to a third embodiment, FIG. 12B is a resultant image of ditherprocessing using the threshold matrix, and FIG. 12C illustrates an imagewhich is the resultant image subject to rotation processing.

FIG. 13A illustrates a threshold matrix with rotation toleranceaccording to the third embodiment, FIG. 13B is a resultant image ofdither processing using the threshold matrix, and FIG. 13C illustratesan image which is the resultant image subject to rotation processing.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments for implementing the present invention will bedescribed with reference to the accompanying drawings. Configurationsdescribed in the following embodiments are illustrative only, and notrestrictive.

First Embodiment

A multi-function printer (MFP) having a color printing function by anelectrophotographic system using toner of four colors (CMYK) will bedescribed as an example in the present embodiment, but the image formingapparatus to which the present invention is applicable is not limited tothe same. The present invention is applicable also to a printer having amonochrome printing function, or an image forming apparatus employinganother recording system, such as an inkjet recording system.

FIG. 1 is a block diagram illustrating a basic configuration of a MFP. AMFP 100 includes a control unit 101, an operation display unit 102, animage processing unit 103, a paper feeding unit 104, an engine I/F 105,a printer engine 106, a network I/F 107, a scanner I/F 108, a scanner109, and a bus 110.

The control unit 101 is a module for controlling the entire MFP 100, andincludes a CPU, ROM, RAM, etc. The control unit 101 performs varioustypes of processing based on a program stored in the ROM. A system bootprogram, a control program of the printer engine 106, and various typesof data, such as character data and character code information, isstored in the ROM. The RAM is used as a work area of the CPU in whichvarious programs are developed, and is used also as a temporary storageof received image data.

The operation display unit 102 is constituted by a liquid crystaldisplay having a touch panel function, for example, and performs varioustypes of display under the control of the control unit 101. Theoperation display unit 102 displays, for example, an operation screenfor receiving designation of print conditions, including layout,enlargement, reduction, and rotation at the time of printing, andreceives an input operation from a user. The operation display unit 102is used also to display information about various setting states of theMFP 100 and processing being performed (e.g., an error status). Further,the operation display unit 102 is used by the user to make variousinstructions, such as change of settings.

The image processing unit 103 performs necessary image processing withrespect to the input image data and generates image data in a formatcompatible with that of the printer engine 106. Details thereof will bedescribed later.

The paper feeding unit 104 includes plural sheet feed cassettes, andsupplies paper sheets to the printer engine 106 under the control of thecontrol unit 101. Each of the sheet feed cassettes may accommodate papersheets of different size (e.g., A4 and A3), or paper sheets of the samesize but different directions (e.g., A4, a landscape (horizontal) and aportrait (vertical) directions).

The engine I/F 105 is an interface on which a command etc. forcontrolling the printer engine 106 is output and input. The printerengine 106 receives image data generated in a predetermined format fromthe image processing unit 103, and forms an image on a surface of apaper sheet fed from the paper feeding unit 104. In electrophotography,printing to a surface of paper which is a recording medium is completedthrough processings of exposure, development, transfer, and fixing. Thenetwork I/F 107 is an interface for connecting the MFP 100 to anunillustrated network. The MFP 100 receives image data which becomes aprinting target from a host computer (not illustrated) via the networkand the network I/F 107. The scanner I/F 108 is an interface to whichthe scanner 109 is connected, and transfers the image data read by thescanner 109 to the image processing unit 103. The bus 110 functions as adata path among the components described above. The components of theMFP 100 described above are illustrative only, and the MFP 100 mayinclude, for example, HDD as a mass storage device for storing varioustypes of data, etc., an external interface for connecting to an externalapparatus, such as a camera and a portable terminal, etc.

Next, the image processing unit 103 will be described in detail. FIG. 2is a block diagram illustrating an internal configuration of the imageprocessing unit 103. The image processing unit 103 includes a PDLprocessing unit 201, a scanned image processing unit 202, and an outputimage processing unit 203. These function units which constitute theimage processing unit 103 are implemented when the CPU executes apredetermined program stored in the ROM. A part or all of these functionunits may be implemented by a dedicated IC.

First, print processing (PDL printing) based on image data input fromthe host computer will be described. In the host computer, digitaldocument data, such as a page layout document, a word processordocument, and a graphic document, is generated by various applications.The generated digital document data is converted into drawing commanddata called a page description language (PDL) (hereinafter, “PDL data”)by a printer driver. The PDL data typically includes drawing data foreach attribute, such as an image, graphic and text, as well asinformation about print setting, such as print resolution, the number ofprint copies, a page layout, and a printing order. The PDL data input inthe MFP 100 via the network I/F 107 is transmitted to the PDL processingunit 201 in the image processing unit 103.

The PDL processing unit 201 performs analysis processing of the receivedPDL data, generates an object of a drawing target, and performsrasterizing processing to generate data in a bit map format in which animage is expressed by colored dots (hereafter, referred to as “rasterimage data.”) At this time, information about the above-described printsetting included in the PDL data is also extracted. The generated rasterimage data and the extracted print setting information are transmittedto the output image processing unit 203.

The output image processing unit 203 performs image processing withrespect to the raster image data for print processing, such as colorconversion processing, density correction processing, rotationprocessing, and pseudo halftone processing, based on the print settinginformation. The image data converted into a format compatible with thatof the printer engine 106 by pseudo-halftoning processing is transmittedto the printer engine 106 via the engine I/F 105 and is printed. Inelectrophotography, a rotating photoconductive drum is irradiated withlaser (exposure), charged toner is made to adhere to the photoconductivedrum (development), a toner image on the photoconductive drum istransferred to a paper sheet via a transfer belt (transfer), and thepaper sheet bearing the toner is pressed with heat and pressure(fixing), whereby printing is completed.

The PDL print processing is completed in the process described above.

Next, print processing (copy printing) based on the image data obtainedwith the scanner 109 will be described. The scanner 109 optically scansa document placed on an unillustrated document table or an automaticdocument feeder (ADF), measures intensity of reflected light andtransmitted light, and performs analog-to-digital conversion to obtainthe raster image data. The raster image data obtained here typically isimage data of a RGB color space. The raster image data obtained by thescanner 109 is transmitted to the raster image processing unit 202.

The raster image processing unit 202 performs predetermined imageprocessing, such as shading correction, in-line correction, and colorcorrection, with respect to the received raster image data. The rasterimage data subject to the predetermined image processing is transmittedto the output image processing unit 203. The output image processingunit 203 performs the above-described pseudo-halftoning processing etc.,and then the printer engine 106 performs the print processing.

The copy printing processing is completed in the process describedabove.

There is a case where not PDL data but raster image data in the form ofJPEG or TIFF is input from the host computer. In that case, the imagedata, even if it is transmitted from the host computer, is transmittedto the raster image processing unit 202.

Next, the output image processing unit 203 will be described in detail.FIG. 3 is a block diagram illustrating an internal configuration of theoutput image processing unit 203. The output image processing unit 203includes a color conversion processing unit 301, a density correctionprocessing unit 302, a pseudo halftone processing unit 303, a thresholdmatrix holding unit 304, an image storage unit 305, a rotation controlunit 306, and a rotation processing unit 307.

The color conversion processing unit 301 performs color conversionprocessing for converting the color space of the raster image data inputfrom the PDL processing unit 201 or the raster image processing unit 202(here RGB) into the color space corresponding to the toner of 4 colorsused in the printer engine 106 (here CMYK). This color conversionprocessing is performed using, for example, a color conversion table,etc. in which RGB values and CMYK values are correlated.

The density correction processing unit 302 performs processing tocorrect density depending on density characteristics inherent in theprinter engine 106 (gamma correction processing) with respect to theraster image data of the CMYK color space which has been subject to thecolor conversion processing. The density correction processing isperformed by using a density correction table in which input densitylevels and output density levels are correlated for each color of CMYK,or by obtaining the density by function calculation.

The pseudo halftone processing unit 303 performs pseudo halftoneprocessing with respect to the raster image data which has been subjectto density correction, and generates image data expressed by halftonedots (i.e., halftone image data, hereinafter, “HT image data”). Theprinter engine 106 can typically output only at a small number of tones,such as 2, 4 and 16. Therefore, the pseudo halftone processing isperformed typically by error diffusion or dithering to enable stablehalftone expression also in the printer engine 106 which can output onlyat a small number of tones. It is presumed that pseudo-halftoningprocessing is performed by dithering in the present embodiment. Here,dithering will be described briefly. In dithering, a threshold matrix inwhich different thresholds are disposed in a matrix of a predeterminedsize is used. This threshold matrix is sequentially developed in a tileshape on the input image data, and multi-value input image data (inputpixel values) is compared with the threshold. If the input pixel valueis larger than the threshold, the pixel is turned on, and if the inputpixel value is smaller than the threshold, the pixel is turned off. Inthis manner, a pseudo halftone image (a halftone image) is expressed. Atthat time, the threshold matrix to be applied in accordance with anattribute of an object is changed. For example, a threshold matrix forlow screen ruling is used for an object which gives priority tocontinuity of tone, such as a photograph, and a threshold matrix forhigh screen ruling is used for an object which gives priority toresolution, such as characters. These threshold matrices are prepared inadvance, held by the threshold matrix holding unit 304, and referred towhen required. Details of the threshold matrix held by the thresholdmatrix holding unit 304 will be described later. The HT image datagenerated by the pseudo halftone processing unit 303 is stored in theimage storage unit 305 (constituted by the RAM etc.) in the output imageprocessing unit 203.

The rotation control unit 306 controls the rotation processing unit 307.Specifically, the rotation control unit 306 determines whether toperform rotation processing and, if the rotation processing isperformed, determines how many degrees the dot is to be rotated based onthe print setting information and out-of-paper-sheet information, andtransmits the determination result to the rotation processing unit 307as a rotation control signal. Hereinafter, cases where it is determinedthat rotation processing be necessary and examples of rotation angleswill be provided.

-   -   a case where vertical inversion in front and rear sides is        designated in double-sided printing: 180°    -   a case where N in 1 printing and bookbinding printing are        designated: 90°    -   a case where a rotation sorting function is effective: 90° or        270°    -   a case where an automatic cassette change function is effective        (e.g., a sheet feed cassette for A4R is changed to a sheet feed        cassette for A4 upon running out of paper sheets): 90° or 270°.

Here, the double-sided printing means printing images on both sides of apaper sheet, N in 1 printing means printing plural pages on a papersheet, and bookbinding printing means allocating pages and printingimages to make a two-fold book. The rotation sorting function means afunction to discharge sheets one by one alternately in oppositedirections. If rotation processing is not performed, a rotation controlsignal designating the rotation angle of 0° is generated and istransmitted to the rotation processing unit 307.

The rotation processing unit 307 reads the HT image data from the imagestorage unit 305 in accordance with the rotation control signal from therotation control unit 306, and applies rotation processing. The HT imagedata which has been subject to the rotation processing is transmitted tothe printer engine 106 via the image storage unit 305, and is subject toprint processing.

FIG. 4 is a flowchart illustrating a processing flow in the output imageprocessing unit 203. A series of processing is implemented when acontrol program stored in the ROM is developed in the RAM and the CPUexecutes the control program in the control unit 101. Here, PDL printingin which processes up to printing including rotation processing areperformed without intermission in response to the print command from thehost computer will be described as an example. The following processingwill be started when the input PDL data is converted into raster imagedata and input in the output image processing unit 203 by rasterizationprocessing in the PDL processing unit 201.

In step 401, the color conversion processing unit 301 performs colorconversion processing with respect to data of the 1st page of the inputraster image data to convert from the RGB color space into the CMYKcolor space. The raster image data of the CMYK color space which hasbeen subject to the color conversion processing is transmitted to thedensity correction processing unit 302.

In step 402, the density correction processing unit 302 performs densitycorrection processing for each plate of CMYK with respect to the rasterimage data which has been subject to the color conversion processing.The raster image data which has been subject to density correction istransmitted to the pseudo halftone processing unit 303.

In step 403, the pseudo halftone processing unit 303 performs the pseudohalftone processing by dithering (hereinafter, “dither processing”) foreach plate of CMYK with respect to the raster image data which has beensubject to the density correction processing using the threshold matrixheld in the threshold matrix holding unit 304. Since the number of tonesper pixel of the HT image data is reduced to a small number of tones,such as binary, capacitance of the image storage unit 305 and aprocessing speed during storage are advantageous as compared with a casewhere image data before being subject to dither processing is stored asit is.

In step 404, the HT image data generated by the dither processing instep 403 is temporarily stored in the image storage unit 305.

In step 405, the rotation control unit 306 determines whether rotationprocessing is necessary. If double-sided printing, N in 1 printing, andbookbinding printing are designated in the above-described examples, orif the rotation sorting function is effective, whether rotationprocessing is necessary is determined based on the print settinginformation. In this case, information about print setting included inthe PDL data is referred to in the PDL printing, and information aboutthe print setting set on the operation display unit 103 is referred toif an image read with a scanner is to be printed. If the automaticcassette change function is effective, whether rotation processing isnecessary is determined in response to out-of-paper-sheet informationtransmitted from the paper feeding unit 104. The determination resultbecomes a rotation control signal representing either of 4 directions ona 90° basis about 0° which means no rotation (0°, 90°, 180°, and 270°clockwise), and is transmitted to the rotation processing unit 307.

In step 406, the rotation processing unit 307 rotates HT image datastored in the image storage unit 305 in accordance with the rotationcontrol signal.

In step 407, the HT image data subject to rotation processing asnecessary is output to the printer engine 106.

In step 408, it is determined whether the input raster image dataincludes an unprocessed page. If an unprocessed page is included, theroutine returns to step 401, where processing is continued with respectto the next page as a process target page. If all the pages have beenprocessed, the process leaves the loop.

The processing flow in the output image processing unit 203 has beendescribed. In the present embodiment, a case where each processing toprint output including rotation processing is performed withoutintermission is described as an example, but this is not restrictive.For example, the HT image data may be generated and stored using a BOXsaving function first, and then, at desirable timing by the user, it maybe determined whether rotation of the saved HT image data is necessaryand the saved HT image data may be rotated before being printed out. Inthis case, the HT image data is stored generally in a mass storagedevice, such as unillustrated HDD instead of the RAM in the output imageprocessing unit 203.

Next, the threshold matrix used in the dither processing in the pseudohalftone processing unit 303 will be described in detail. In the presentembodiment, it is assumed that the image data subject to ditherprocessing is rotated on a 90° basis. First, a conventional thresholdmatrix, a resultant image of dither processing (a HT image), and a HTimage subject to the rotation processing are described with reference toFIGS. 5A to 5C.

FIG. 5A is an example of an orthodox threshold matrix having a screenangle of 72° and the screen ruling of 190 lpi when applied to image dataof 1200 dpi. The threshold matrix illustrated in FIG. 5A is constitutedby four submatrices. Numerals in the submatrices (0 to 39) representthresholds to be compared with the pixel values (i.e., the densityvalues) of the input image. In this case, quantization in 40 tones,i.e., up to maximum density of 40 is possible. Pixels having valuesgreater than the thresholds become ON pixels and form a halftone dot(hereinafter, “dot”). As the size of the dot becomes large, density ofthe image increases. The number of expressible tones can be increased byincreasing the size of the submatrix. For example, it is only necessaryto arrange the numerals of from 0 to 254 in the submatrices for thequantization of 8 bits, i.e., 256 tones. The threshold matrixillustrated in FIG. 5A is developed as one unit on the input image datasequentially in a tile shape without space, and the image data isconverted into a dot pattern. The threshold matrix used as a minimumunit is referred to as a “unit matrix” hereinafter. In the thresholdmatrix illustrated in FIG. 5A, dots obtained by applying the thresholdmatrix to flat input image data all of which pixel values (densityvalues) are 5 are illustrated in gray. In this case, the pixel valuesexceed the threshold at positions where the thresholds are smaller than5, and four dots 501 to 504 each consisting of five ON pixels appear.FIG. 5B illustrates a HT image obtained by applying the threshold matrixillustrated in FIG. 5A to the flat input image data all of which pixelvalues are 5 (a reference HT image of rotation processing; hereinafter,referred to as a “reference HT image.”) FIG. 5C illustrates an imageobtained by rotating the reference HT image of FIG. 5B 90° clockwise.When FIGS. 5B and 5C are compared, it turns out that the shape of thedot has been changed from horizontally long to vertically long. This maycause a change in density on a paper sheet obtained as a printed resultbefore and after the rotation processing for the reason described in thebackground art. The problem of the change in density accompanying therotation processing is caused due to a change in the shape of the formeddot, and thus the problem occurs only in a density region where theshape of the dot is not symmetric in vertical and horizontal directions.That is, in a flat input image all of which pixel values is 4, since asquare dot consisting of four ON pixels is formed when the thresholdmatrix of FIG. 5A is applied, the shape of the dot is not changed due torotation processing on a 90° basis. Therefore, the problem of change indensity does not occur.

Next, a threshold matrix with rotation tolerance configured not to causea change in density before and after the rotation processing accordingto the present embodiment will be described. FIG. 6A is an example of athreshold matrix with rotation tolerance according to the presentembodiment. The threshold matrix is the same in size, screen ruling, andscreen angle as those of the threshold matrix illustrated in FIG. 5A. Inthis case, quantization in 40 tones, i.e., up to a maximum density of 40is possible. Four dots 601 to 604 obtained by applying the thresholdmatrix to the flat input image data all of which pixel values are 5 areillustrated in gray in the same manner as in FIG. 5A. If the thresholdmatrix of FIG. 6A is applied, the formed four dots 601 to 604 have thesame shape but are oriented in four different directions at 90° (0°,90°, 180° and 270°). In the present embodiment, the threshold matrix inwhich each threshold is arranged in the submatrix so that dots orientedin four different directions are formed is defined as a unit matrix.FIG. 6B is a resultant image obtained by applying the threshold matrix(i.e., the unit matrix) of FIG. 6A to the flat input image data all ofwhich pixel values are 5 (a reference HT image). FIG. 6C illustrates animage obtained by rotating the reference HT image of FIG. 6B 90°clockwise. In this case, for example, even if a certain dot oriented in0° is rotated 90°, another dot oriented in 270° is rotated 90° to beoriented in 0°, whereby the dot pattern to appear in the entire image isunchanged. The dots with respective orientations are replaced due torotation and a relationship between the dots is unchanged. Therefore, achange in density on a printing paper sheet does not occur depending onwhether rotation processing is performed (i.e., between a reference HTimage and a 90°-rotated HT image).

FIGS. 7A to 7C and 8A to 8C illustrate examples in which a conventionalthreshold matrix and the threshold matrix with rotation toleranceaccording to the present embodiment are applied to a flat input imageall of which pixel values are 10. Specifically, FIG. 7A illustrates thesame conventional threshold matrix as that illustrated in FIG. 5A, FIG.7B is a resultant image obtained by applying the threshold matrix ofFIG. 7A (a reference HT image), and FIG. 7C is the reference HT imagerotated 90°. Similarly, FIG. 8A illustrates the same threshold matrixwith rotation tolerance as that illustrated in FIG. 6A, FIG. 8B is aresultant image obtained by applying the threshold matrix of FIG. 8A (areference HT image), and FIG. 8C is the reference HT image rotated 90°.In any case, the size of the dot (i.e., a dot area) is twice as thoseillustrated in FIGS. 5A to 5C and 6A to 6C. Regarding the formed dotpattern, in the case of FIGS. 7A to 7C where the conventional thresholdmatrix is used, the shapes of all the formed dots have been changed fromvertically long to horizontally long due to rotation processing. In thecase of FIGS. 8B and 8C where the threshold matrix with rotationtolerance is used, in both the reference HT image illustrated in FIG. 8Band the 90°-rotated HT image illustrated in FIG. 8C, the same number ofvertically long dots and horizontally long dots exist, leaving nodifference therebetween. At this density (pixel value: 10), since eachdot is rotationally symmetric at 180°, orientations of the dots are only0° and 90°. Therefore, uniformity of the dots on the surface is furtherincreased. As the uniformity becomes higher, an output result with highimage granulation and high quality can be obtained.

Here, if the total number of pixels which constitute the dot is an evennumber, it is always possible to form a dot shape of rotationalsymmetric at 180°. Exemplary control of a threshold matrix so that ashape of a dot becomes rotationally symmetric at 180° when the totalnumber of pixels constituting the dot is an even number will bedescribed. FIG. 9 illustrates submatrices 901 to 920 of 6×6 pixelsconstituting the threshold matrix in this case. The size of the dotillustrated in gray in each submatrix becomes larger as it approachesfrom 901 toward 920 (i.e., the number of ON pixels increases one byone), and the density increases. In the submatrix in which the totalnumber of the pixels constituting the dot is an even number (902, 904,906, 908, 910, 912, 914, 916, 918 and 920), the shape of the dot isalways rotationally symmetric at 180°. In addition, since the dot in thesubmatrices of 904 and 916 is rotational symmetric at 90°, perfectuniformity of dot is obtained regarding density at this time (i.e.,density values 4 and 16) before and after rotation of the image. In thisexample, when the total number of the pixels constituting a dot becomesan odd number (each of the submatrices of 903, 905, 907, 909, 911, 913,915, 917 and 919), the four dots constituted by the unit matrix areoriented in four different directions at 90°. When the total number ofthe pixels constituting a dot becomes an even number, the four dotsconstituted by the unit matrix are oriented in two different directionsat 180° or one direction.

The threshold matrix with rotation tolerance made under such a conditionis held for each color of CMYK in the above-described threshold matrixholding unit 304. It is typically tried to suppress generation of moirebetween colors by using a threshold matrix with screen ruling or screenangles different for each color of CMYK. However, since suppression ofgeneration of moire is not a feature of the present invention, specificdescription thereof is omitted.

Although the HT image data subject to dither processing is described asa 2-tone image representing ON and OFF on a pixel unit in the presentembodiment, the HT image data subject to dither processing may be a4-tone image, a 16-tone image, etc.

As described above, according to the present embodiment, the thresholdmatrix is configured such that the dots formed by dither processing areoriented in four different directions basically on a 90° basis. That is,when the shape of the dot is not rotational symmetric on a 90° basis,the dots are oriented in the four directions as described above, whenthe dot is rotational symmetric on a 180° basis, the dots are orientedin the two directions, and when the dot is rotational symmetric on a 90°basis, the dots are oriented in one direction (i.e., the same shape seenfrom any direction). Therefore, in dither processing, HT image data withreduced occurrence of a change in density accompanying rotationprocessing can be generated while keeping continuity of tone.

Second Embodiment

In the first embodiment, the rotation angles in the rotation processingare four on a 90° basis including 0°. This is because, rotationprocessing at 90° (or 270°) is required as the orientation is changedfrom the landscape to the portrait in rotation sorting or automaticcassette change. However, when it is limited to the double-sidedprinting, only 180° rotation processing is required. In the double-sidedprinting, whether an image is printed on the back side in the samevertical direction as that of the front side depends on the usage of theprinted matter. For example, if binding positions of output plural pagesare located on a side in the longitudinal direction, it is desirablethat images on the back side and the front side are oriented in the samevertical direction, whereas if the binding positions are located on anupper side in the width direction, it is desirable that images on theback side and the front side are oriented in the opposite verticaldirections. Neither of the cases requires rotation processing of 90° or270°. Then, an embodiment in which the rotation angle of the rotationprocessing is limited to 180° and a threshold matrix with tolerance torotation of 180° is used will be described as a second embodiment.

A basic configuration of the MFP 100, a configuration of the outputimage processing unit 203, etc., which are the same as those of thefirst embodiment will not be described. A threshold matrix used fordither processing which is a difference between the first embodiment andthe second embodiment will be described mainly.

FIG. 10A is an example of a threshold matrix with rotation tolerance at180° according to the present embodiment. The threshold matrix is thesame in size, screen ruling, and screen angle as those of the thresholdmatrix illustrated in FIG. 6A of the first embodiment. In this case,quantization in 40 tones, i.e., up to a maximum density of 40 ispossible. Four dots 1001 to 1004 obtained by applying the thresholdmatrix to the flat input image data all of which pixel values are 5 areillustrated in gray in the same manner as in FIG. 6A. If the thresholdmatrix of FIG. 10A is applied, each of the formed four dots 1001 to 1004are the same in shape as in the first embodiment. The diagonallyopposite 2 sets of dots (i.e., the dots 1001 and 1003, and dots 1002 and1004) are oriented in the opposite directions (i.e., 0° and 180°). Inthe present embodiment, the threshold matrix in which a threshold isarranged in each submatrix so that such a dot is formed is defined as aunit matrix. FIG. 10B is a resultant image obtained by applying thethreshold matrix (i.e., the unit matrix) of FIG. 10A to the flat inputimage data all of which pixel values are 5 (a reference HT image). FIG.10C illustrates an image obtained by rotating the reference HT image ofFIG. 10B 180° clockwise. In this case, for example, even if a dotoriented in 0° is rotated 180°, another dot oriented in 180° is rotated180° to be oriented in 0°, whereby the dot pattern to appear isunchanged. The dots with respective orientations are replaced due torotation, no change in density on a paper sheet which is print outputoccurs whether the dot is rotated.

FIGS. 11A to 11C illustrate an example in which a threshold matrix withrotation tolerance according to the present embodiment is applied to aflat input image all of which pixel values are 10. Specifically, FIG.11A illustrates the same threshold matrix as that illustrated in FIG.10A, FIG. 11B is a reference HT image obtained by applying the thresholdmatrix of FIG. 11A, and FIG. 11C is the reference HT image rotated 180°.The size of the dot (i.e., a dot area) becomes twice the size of the dotillustrated in FIGS. 10B and 10C and the shape of the dot coincidescompletely before and after the rotation. Therefore, in the case of thepresent embodiment, since it is assumed that the rotation angle islimited to 180°, uniformity of the shape of the dot is further increasedand a printed result with higher granularity can be obtained.

Third Embodiment

As described above, in an electrophotographic printing process, forexample, a relationship between a laser scanning direction and arotation direction of a photoconductive drum is exchanged is when arotation angle is 90° and 270°. A change in density easily occursbetween a printed result subject to rotation processing at these anglesand a printed result not subject to rotation processing, and a degree ofchange is large. Conversely, a change in density does not occur easilyif the rotation angle is set so that a relationship between the laserscanning direction and the rotation direction of the photoconductivedrum is not changed, and a degree of change is small even if it occurs.Then, an embodiment in which it is considered that no change in densityexists between 0° and 180° and between 90° and 270°, in which a rotationangle is set to be on a 90° basis (in four directions) as in the firstembodiment, and orientations of dots to be formed are limited to be on a180° basis (in two directions) will be described as a third embodiment.

A basic configuration of the MFP 100, a configuration of the outputimage processing unit 203, etc., which are the same as those of thefirst embodiment will not be described. A threshold matrix used fordither processing which is a difference between the first embodiment andthe second embodiment will be described mainly.

FIG. 12A is an example of a threshold matrix with rotation toleranceaccording to the present embodiment. The threshold matrix is the same insize, screen ruling, and screen angle as those of the threshold matrixillustrated in FIG. 6A of the first embodiment. In this case,quantization in 40 tones, i.e., up to a maximum density of 40 ispossible. Four dots 1201 to 1204 obtained by applying the thresholdmatrix to the flat input image data all of which pixel values are 5 areillustrated in gray in the same manner as in FIG. 6A. If the thresholdmatrix of FIG. 12A is applied, each of the formed four dots 1201 to 1204are the same in shape in the same manner as in the first embodiment. Inthe threshold matrix of the present embodiment, the diagonally opposite2 sets of dots (i.e., the dots 1201 and 1203, and dots 1202 and 1204)are oriented in the opposite directions (i.e., 0° and 90°). In thepresent embodiment, the threshold matrix in which a threshold isarranged in each submatrix so that such a dot is formed is defined as aunit matrix. FIG. 12B is a resultant image obtained by applying thethreshold matrix (i.e., the unit matrix) of FIG. 12A to the flat inputimage data all of which pixel values are 5 (a reference HT image). FIG.12C illustrates an image obtained by rotating the reference HT image ofFIG. 12B 90° clockwise. In this case, the dots oriented in twodirections are replaced due to rotation to obtain a dot patternconstituted by either of the same or 180°-rotated dots. If it is assumedthat no change in density occurs even if the dots rotate 180°, no changein density will occur on a paper sheet which is print output whether thedots are rotated.

FIGS. 13A to 13C illustrate an example in which a threshold matrix withrotation tolerance according to the present embodiment is applied to aflat input image all of which pixel values are 10. Specifically, FIG.13A illustrates the same threshold matrix as that illustrated in FIG.12A, FIG. 13B is a resultant image obtained by applying the thresholdmatrix of FIG. 13A (a reference HT image), and FIG. 13C is the referenceHT image rotated 90°. The size of the dot (i.e., a dot area) becomestwice the size of the dot illustrated in FIGS. 12A to 12C and the shapeof the dot coincides completely before and after the rotation.Uniformity of the shape of the dot is further increased and a printedresult with higher granularity can be obtained in the same manner as inthe second embodiment.

Other Embodiments

The present invention is applicable also to a process in which a programthat performs one or more functions of the above-described embodimentsis supplied to a system or an apparatus via a network or a storagemedium, and one or more processors in a computer of the system or theapparatus read and execute the program. Further, the present inventionis implementable in a circuit having one or more functions (e.g., ASIC).

Advantageous Effects of Invention

According to the present invention, even if image data which has beensubject to pseudo-halftoning processing is rotated at the time of printoutput, it is possible to reduce a change in density while keepingfavorable continuity of tone.

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-227583, filed Nov. 20, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image processing apparatus which performsimage processing for print processing, comprising: a pseudo halftoneprocessing unit configured to perform pseudo halftone processing bydithering with respect to an input image, and generate a halftone imageconstituted by plural dots, and a threshold matrix holding unitconfigured to hold a threshold matrix used for the pseudo halftoneprocessing, wherein in the threshold matrix, thresholds are arrangedsuch that the halftone image has a density region in which dots to beformed depending on a density of the input image are not rotationalsymmetric on a 90° basis, and orientations of the dots differ from eachother in the density region.
 2. The image processing apparatus accordingto claim 1, wherein in the threshold matrix, the thresholds are arrangedsuch that the dots are oriented in four different directions on a 90°basis in the density region in which the dots are not rotationalsymmetric on a 90° basis, and the thresholds are arranged such that thedots are oriented in two directions at 180° in a density region in whichthe dots are rotational symmetric on a 180° basis in the density regionin which the dots are not rotational symmetric on a 90° basis.
 3. Theimage processing apparatus according to claim 2, wherein in thethreshold matrix, if the total number of pixels constituting each dot isan even number, the thresholds are arranged such that each dot isrotational symmetric at 180°.
 4. The image processing apparatusaccording to claim 1, further comprising a rotation processing unitconfigured to perform rotation processing with respect to the halftoneimage, wherein a rotation angle in the rotation processing unit iseither 90°, 180° or 270°.
 5. The image processing apparatus according toclaim 4, further comprising a rotation control unit configured todetermine the rotation angle of the rotation processing on a 90° basisbased on information about the print processing, and control therotation processing.
 6. The image processing apparatus according toclaim 5, wherein the information about the print processing is printsetting information to designate a print condition of the input image,and if double-sided printing is designated and vertical inversion onfront and back sides is designated by the print setting information, therotation control unit controls the rotation processing unit such thatthe rotation angle becomes 180°.
 7. The image processing apparatusaccording to claim 5, wherein the information about the print processingis print setting information to designate a print condition of the inputimage, and if N in 1 printing or bookbinding printing is designated bythe print setting information, the rotation control unit controls therotation processing unit such that the rotation angle becomes 90°. 8.The image processing apparatus according to claim 5, wherein theinformation about the print processing is print setting information todesignate a print condition of the input image, and if a rotationsorting function is set to be effective by the print settinginformation, the rotation control unit controls the rotation processingunit such that the rotation angle becomes 90° or 270°.
 9. The imageprocessing apparatus according to claim 5, wherein the information aboutthe print processing is information representing running out of papersheet in a case where an automatic cassette change function is set to beeffective in a printing apparatus which performs the print processing,the rotation control unit controls the rotation processing unit suchthat the rotation angle becomes 90° or 270° in response to theinformation representing out of paper sheet.
 10. The image processingapparatus according to claim 1, wherein in the threshold matrix, thethresholds are arranged such that diagonally opposite 2 sets of dots areoriented in two directions on a 180° basis.
 11. The image processingapparatus according to claim 10, further comprising a rotationprocessing unit configured to perform rotation processing with respectto the halftone image data, wherein a rotation angle in the rotationprocessing unit is 180°.
 12. The image processing apparatus according toclaim 1, wherein in the threshold matrix, the thresholds are arrangedsuch that the dots are oriented in two directions on a 180° basis in thedensity region in which the dots are not rotational symmetric on a 90°basis.
 13. The image processing apparatus according to claim 12, furthercomprising a rotation processing unit configured to perform rotationprocessing with respect to the halftone image, wherein a rotation anglein the rotation processing unit is either 90°, 180° or 270°.
 14. Amethod for performing pseudo halftone processing by dithering withrespect to an input image, and generating a halftone image constitutedby plural dots, wherein in a threshold matrix used for the pseudohalftone processing, thresholds are arranged such that the halftoneimage has a density region in which dots to be formed depending on adensity of the input image are not rotational symmetric on a 90° basis,and orientations of the dots differ from each other in the densityregion.
 15. A non-transitory computer readable storage medium storing aprogram for causing a computer to perform a method for controlling animage processing apparatus which performs pseudo halftone processing bydithering with respect to an input image, and generates a halftone imageconstituted by plural dots, the control method comprising the steps of;in a threshold matrix used for the pseudo halftone processing,thresholds are arranged such that the halftone image has a densityregion in which dots to be formed depending on a density of the inputimage are not rotational symmetric on a 90° basis, and orientations ofthe dots differ from each other in the density region.